java swing. introduction to swing a gui (graphical user interface) is a windowing system that...

Java Swing

Introduction to Swing• A GUI (graphical user interface) is a windowing system that

interacts with the user

• The Java AWT (Abstract Window Toolkit) package is the original Java package for doing GUIs

• The Swing package is an improved version of the AWT– However, it does not completely replace the AWT– Some AWT classes are replaced by Swing classes, but other AWT

classes are needed when using Swing• Swing GUIs are designed using a form of object-oriented

programming known as event-driven programming

Events

• Event-driven programming is a programming style that uses a signal-and-response approach to programming

• An event is an object that acts as a signal to another object know as a listener

• The sending of an event is called firing the event– The object that fires the event is often a GUI component,

such as a button that has been clicked

Listeners

• A listener object performs some action in response to the event– A given component may have any number of

listeners– Each listener may respond to a different kind of

event, or multiple listeners may respond to the same events

Exception Objects

• Where have we seen this “Even-Driven” programming before?

• An exception object is an event– The throwing of an exception is an example of firing

an event

• The listener for an exception object is the catch block that catches the event

Event Handlers

• A listener object has methods that specify what will happen when events of various kinds are received by it– These methods are called event handlers

• The programmer using the listener object will define or redefine these event-handler methods

Event Firing and an Event Listener

Event-Driven Programming

• Event-driven programming is very different from most programming seen up until now– So far, programs have consisted of a list of statements

executed in order– When that order changed, whether or not to perform

certain actions (such as repeat statements in a loop, branch to another statement, or invoke a method) was controlled by the logic of the program


• In event-driven programming, objects are created that can fire events, and listener objects are created that can react to the events

• The program itself no longer determines the order in which things can happen– Instead, the events determine the order


• In an event-driven program, the next thing that happens depends on the next event (e.g., which button or menu will be selected by the user)

• In particular, methods are defined that will never be explicitly invoked in any program– Instead, methods are invoked automatically when an event

signals that the method needs to be called

A SIMPLE WINDOW

A First Swing Demonstration (Part 4 of 4)

A Simple Window• A simple window can consist of an object of the JFrame

class– A JFrame object includes a border and the usual three buttons for

minimizing, changing the size of, and closing the window– The JFrame class is found in the javax.swing package

JFrame firstWindow = new JFrame();

• A JFrame can have components added to it, such as buttons, menus, and text labels– These components can be programmed for action

firstWindow.add(endButton);– It can be made visible using the setVisible method

firstWindow.setVisible(true);


Create a JFrame object


Set window size with numbers of pixels

Pixels and the Relationship between Resolution and Size

• A pixel is the smallest unit of space on a screen– Both the size and position of Swing objects are measured in

pixels– The more pixels on a screen, the greater the screen resolution

• A high-resolution screen of fixed size has many pixels– Therefore, each one is very small

• A low-resolution screen of fixed size has fewer pixels– Therefore, each one is much larger

• Therefore, a two-pixel figure on a low-resolution screen will look larger than a two-pixel figure on a high-resolution screen


Create a button object with text


Add the button to the JFrame object

Buttons

• A button object is created from the class JButton and can be added to a JFrame– The argument to the JButton constructor is the string

that appears on the button when it is displayed

JButton endButton = new JButton("Click to end program.");firstWindow.add(endButton);


Associate an Even Lister


Define Even Listener


Make the JFrame object visible

Pitfall: Forgetting to Program the Close-Window Button

• The following lines from the FirstSwingDemo program ensure that when the user clicks the close-window button, nothing happensfirstWindow.setDefaultCloseOperation( JFrame.DO_NOTHING_ON_CLOSE);

• If this were not set, the default action would be JFrame.HIDE_ON_CLOSE– This would make the window invisible and inaccessible, but

would not end the program– Therefore, given this scenario, there would be no way to click

the "Click to end program" button• Note that the close-window and other two

accompanying buttons are part of the JFrame object, and not separate buttons

Change a few places and see what will happen?

Action Listeners and Action Events• Clicking a button fires an event• The event object is "sent" to another object called a listener

– This means that a method in the listener object is invoked automatically

– Furthermore, it is invoked with the event object as its argument• In order to set up this relationship, a GUI program must do

two things1. It must specify, for each button, what objects are its listeners, i.e., it

must register the listeners2. It must define the methods that will be invoked automatically when

the event is sent to the listener


Define Even Listener

Action Listeners and Action Events

EndingListener buttonEar = new EndingListener();endButton.addActionListener(buttonEar);

• Above, a listener object named buttonEar is created and registered as a listener for the button named endButton– Note that a button fires events known as action events,

which are handled by listeners known as action listeners


• Different kinds of components require different kinds of listener classes to handle the events they fire

• An action listener is an object whose class implements the ActionListener interface– The ActionListener interface has one method

heading that must be implementedpublic void actionPerformed(ActionEvent e)

Action Listeners and Action Eventspublic void actionPerformed(ActionEvent e){ System.exit(0);}

• The EndingListener class defines its actionPerformed method as above– When the user clicks the endButton, an action event is sent

to the action listener for that button– The EndingListener object buttonEar is the action

listener for endButton– The action listener buttonEar receives the action event as

the parameter e to its actionPerformed method, which is automatically invoked

– Note that e must be received, even if it is not used

Pitfall: Changing the Heading for actionPerformed

• When the actionPerformed method is implemented in an action listener, its header must be the one specified in the ActionListener interface– It is already determined, and may not be changed– Not even a throws clause may be addedpublic void actionPerformed(ActionEvent e)

• The only thing that can be changed is the name of the parameter, since it is just a placeholder– Whether it is called e or something else does not matter, as long as it

is used consistently within the body of the method

A Better Version of the Previous indow

The Normal Way to Define a JFrame (Part 1 of 4)

What Can be Summarized?Derive a new container class from, say, JFrame

In the derived classAdd the interface objects, such as buttons and othersAssociate a listener object for each interface object

Define listener classes to handle possible events fired by the interface objects added in the window

In the main function Create the object of the derived window class Define a constructor that sets up the title and size of the windowLaunch the interface by setting it as visible

Labels

• A label is an object of the class JLabel– Text can be added to a JFrame using a label– The text for the label is given as an argument when the JLabel is created

– The label can then be added to a JFrameJLabel greeting = new JLabel("Hello");add(greeting);

Color• In Java, a color is an object of the class Color

– The class Color is found in the java.awt package– There are constants in the Color class that represent a number of

basic colors

• JFrame can not be colored directly – Instead, a program must color something called the content pane of

the JFrame– Since the content pane is the "inside" of a JFrame, coloring the

content pane has the effect of coloring the inside of the JFrame– Therefore, the background color of a JFrame can be set using the

following code:getContentPane().setBackground(Color);

A JFrame with Color (Part 4 of 4)

The Color Constants

The color can be customized…

LAYOUT MANAGER

Layout Examples

Containers and Layout Managers

• Multiple components can be added to the content pane of a JFrame using the add method– However, the add method does not specify how these

components are to be arranged

• To describe how multiple components are to be arranged, a layout manager (awt) is used– There are a number of layout manager classes such as BorderLayout, FlowLayout, and GridLayout

– If a layout manager is not specified, a default layout manager is used (e.g., only one widget on the interface)

Flow Layout Managers• The FlowLayout manager is the simplest layout

managersetLayout(new FlowLayout());– It arranges components one after the other, going from left

to right– Components are arranged in the order in which they are

added• Since a location is not specified, the add method has

only one argument when using the FlowLayoutManageradd.(label1);

Border Layout Managers

• A BorderLayout manager places the components that are added to a JFrame object into five regions– These regions are: BorderLayout.NORTH, BorderLayout.SOUTH, BorderLayout.EAST, BorderLayout.WEST, and BorderLayout.Center

• A BorderLayout manager is added to a JFrame using the setLayout method– For example:

setLayout(new BorderLayout());

The BorderLayout Manager (Part 1 of 4)

BorderLayout Regions

Border Layout Managers• The previous diagram shows the arrangement of the five

border layout regions– Note: None of the lines in the diagram are normally visible

• When using a BorderLayout manager, the location of the component being added is given as a second argument to the add methodadd(label1, BorderLayout.NORTH);– Components can be added in any order since their location is specified

• A GridLayout manager arranges components in a two-dimensional grid with some number of rows and columnssetLayout(new GridLayout(rows, columns));– Each entry is the same size– The two numbers given as arguments specify the number of rows and

columns– Each component is stretched so that it completely fills its grid position

– Note: None of the lines in the diagram are normally visible

Grid Layout Managers

Grid Layout Managers• When using the GridLayout class, the method add has only one argumentadd(label1);– Items are placed in the grid from left to right– The top row is filled first, then the second, and so forth– Grid positions may not be skipped

• Note the use of a main method in the GUI class itself in the following example– This is often a convenient way of demonstrating a class

The GridLayout Manager (Part 1 of 4)

Some Layout Managers

What Can be Summarized?Derived a new container class from, say, JFrame

In the derived class• Define a constructor that sets up the title and size of the window• Set up the proper layout of the widgets• Add the interface objects, such as buttons and others• Remember to associate a listener object for each interface object


In the main function• Create the object of the derived window class• Launch the interface by setting it as visible (you can do it in the

constructor of the customized Frame as well)

Layout Summary


• Different kinds of components require different kinds of listener classes to handle the events they fire

• An action listener is an object whose class implements the ActionListener interface– The ActionListener interface has one method

heading that must be implementedpublic void actionPerformed(ActionEvent e){ //TODO}


• To use the customized action listener class, you need to create a listener object first, e.g.,

EndingListener buttonEar = new EndingListener();

• Then, associate this listener with the specific component object, e.g.,

endButton.addActionListener(buttonEar);

The setActionCommand Method

• When a user clicks a button or menu item, an event is fired that normally goes to one or more action listeners– The action event becomes an argument to an actionPerformed

method– This action event includes a String instance variable called the

action command for the button or menu item– The default value for this string is the string written on the button or

the menu item– This string can be retrieved with the getActionCommand method

e.getActionCommand()

The setActionCommand Method

• The setActionCommand method can be used to change the action command for a component– This is especially useful when two or more buttons or

menu items have the same default action command stringsJButton nextButton = new JButton("Next");nextButton.setActionCommand("Next Button");

JMenuItem choose = new JMenuItem("Next");choose.setActionCommand("Next Menu Item");

Listeners as Inner Classes

• Often, instead of having one action listener object deal with all the action events in a GUI, a separate ActionListener class is created for each button or menu item– Each button or menu item has its own unique action

listener– There is then no need for a multiway if-else statement

• When this approach is used, each class is usually made a private inner class

PANNEL

Panel Examples

Panels• A GUI is often organized in a hierarchical fashion,

with containers called panels inside other containers

• A panel is an object of the JPanel class that serves as a simple container– It is used to group smaller objects into a larger component

(the panel)– One of the main functions of a JPanel object is to

subdivide a JFrame or other container

Panels• Both a JFrame and each panel in a JFrame can use

different layout managers– Additional panels can be added to each panel, and each panel

can have its own layout manager– This enables almost any kind of overall layout to be used in a GUI

setLayout(new BorderLayout());JPanel somePanel = new JPanel();somePanel.setLayout(new FlowLayout());

• Note in the following example that panel and button objects are given color using the setBackground method without invoking getContentPane– The getContentPane method is only used when adding color

to a JFrame

Using Panels (Part 7 of 8)

The panels with borderlines example

The Container Class• Any class that is a descendent class of the class Container is

considered to be a container class– The Container class is found in the java.awt package, not in the

Swing library

• Any object that belongs to a class derived from the Container class (or its descendents) can have components added to it

• The classes JFrame and JPanel are descendent classes of the class Container– Therefore they and any of their descendents can serve as a container

The JComponent Class

• Any descendent class of the class JComponent is called a component class– Any JComponent object or component can be

added to any container class object– Because it is derived from the class Container,

a JComponent can also be added to another JComponent

Objects in a Typical GUI

• Almost every GUI built using Swing container classes will be made up of three kinds of objects:

1. The container itself, probably a panel or window-like object

2. The components added to the container such as labels, buttons, and panels

3. A layout manager to position the components inside the container

Hierarchy of Swing and AWT Classes

Tip: Code a GUI's Look and Actions Separately

• The task of designing a Swing GUI can be divided into two main subtasks:

1. Designing and coding the appearance of the GUI on the screen2. Designing and coding the actions performed in response to user

actions

• In particular, it is useful to implement the actionPerformed method as a stub, until the GUI looks the way it shouldpublic void actionPerformed(ActionEvent e){}

What Can be Summarized?Derived a new container class from, say, JFrame

In the derived class• Define a constructor that sets up the title and size of the window• Set up the proper lay out of the outer container• Create inner containers• Set up the proper lay out of each inner containers• Add the interface objects, such as buttons and others, to the

corresponding containers• Remember to associate a listener object for each interface object• Add the containers to the Frame object in order


In the main function• Create the object of the derived window class• Launch the interface by setting it as visible

Menu Examples

Menu Bars, Menus, and Menu Items

• A menu is an object of the class JMenu• A choice on a menu is called a menu item, and is an

object of the class JMenuItem– A menu can contain any number of menu items– A menu item is identified by the string that labels it, and

is displayed in the order to which it was added to the menu

• The add method is used to add a menu item to a menu in the same way that a component is added to a container object

Menu Bars, Menus, and Menu Items

• The following creates a new menu, and then adds a menu item to itJMenu diner = new JMenu("Daily Specials");JMenuItem lunch = new JMenuItem("Lunch Specials");lunch.addActionListener(this);diner.add(lunch);– Note that the this parameter has been registered as

an action listener for the menu item

Nested Menus

• The class JMenu is a descendent of the JMenuItem class– Every JMenu can be a menu item in another

menu– Therefore, menus can be nested

• Menus can be added to other menus in the same way as menu items

Menu Bars and JFrame• A menu bar is a container for menus, typically placed near

the top of a windowing interface

• The add method is used to add a menu to a menu bar in the same way that menu items are added to a menuJMenuBar bar = new JMenuBar();bar.add(diner);

• The menu bar can be added to a JFrame in two different ways1. Using the setJMenuBar method

setJMenuBar(bar);2. Using the add method – which can be used to add a menu bar to a

JFrame or any other container

A GUI with a Menu (Part 7 of 8)

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